Method of fumigation



Patented Sept. 19, 1933 lVIETHOD 0F FUMIGATION George P. Gray, Monrovia, Calif., assignor to American Cyanamid Company, New York,

N. Y., a corporation of Maine No Drawing.

5 Claims.

This invention relates'to fumigation, more particularly to the use of hydrocyanic acid gas either vaporized from liquid hydrocyanic acid or generated from cyanides at the point of use.

It has been customary for many years to fumigate citrus trees with hydrocyanic acid for the eradication of various pests, more especially scale of various types such as red, black, etc. Fumigation has generally consisted in placing a tent over a tree to be fumigated and injecting under the tent a predetermined amount of 'hydrocyanic acid calculated in accordance with certain standard dosage schedules which are well known and in universal use in the citrus regions. These schedules have been standardized and set forth in detail in Farmers Bulletin No. 1321, of the U. S. Department of Agriculture, published at Washington, D. C., July, 1923. A normal dosage is designated as 100, per cent and variations therefrom are providedfor in these schedules, taking into account such factors as time of the year, period of fumigation, age of tree, character of scale and thelike. In referring to regular practice and standard dosage, the schedules in the above bulletin are referred to as the standard.

The hydrocyanic acid injected under the tent vaporizes and diffuses throughout the enclosed space thereby exerting its killing action on the insects. This diffusion has been accomplished through the molecular activity of the gas and convection currents within the space,jwithout the.

assistance of any artificial means. The period of fumigation is normally about 45 minutes to 1 'hour. A fumigation of this kind is generally performed each year with the result that ordinarily more than 90% of the scale is killed but if the kill is low the remainder multiplies so fast that in one years time the number of insects is approximately'the same as before fumigationl In certain of the citrus regions there has been noted a so-called resistant scale which, when treated in the above manner, is not readily killed and the percentage of survivors after a fumigation is relatively high, sometimes from 10 to 35 per cent.

Numerous attempts have been made to eradicate these resistant scale by various modifications of the customary practice, among which are the so-called interval, and the double fumigation methods. The interval fumigation method consists in first subjecting a tree to a partial dosage of hydrocyanic acid for a part of the exposure time and then immediately injecting the remainder of the dose and continuing Application April Serial No. 356,624

the fumigation for theremainder of the normal period of time. The double fumigation method consists in performing two normal fumigations in successionwithout removal of the tents. None ofthese methods has resulted in any appreciable increase in the kill of resistant scale.

I have made a very thorough study of the entire fumigation situation and have conducted many seriesof experiments for the purpose. of determining the reason for the high resistance to fumigation of scale in certain localities and utes for complete vaporization and diffusion of the hydrocyanic acid. At other periods of the year and under less favorable conditions the period of vaporization and dilfusion may be as much as fifteen minutes.

I have discovered that if a scale is subjected to a dosage of hydrocyanic acid which is less than that necessary to kill the same, the scale may become stupefied, its life processes become partially arrested and it is in a dormant state. This state may last 3 hours or more, but in all cases the initial stupefaction of the scale protects it from the effects of normally fatal doses of hydrocyanic acid. This accounts in a large measure for the failure of prior methods to obtain a high kill of resistant? scale and to obtain a uniform kill of the ordinary scale.

My invention seeksto subject the scale to a .toxic concentration of .hydrocyanic acid initially so that there will be no stupefaction of the scale and thereby a muchhigher and more complete kill may be obtained. I have tested out my idea over a long period of time under laboratory and field conditions and have obtained surprising results by a number of methods whereby thehydrocyanic acid was caused to vaporize and be distributed very quickly so as to produce a high initial and uniform concentration which resulted in a high'kill.

In many cases a dosage higher than normal would. be beneficial, but heretofore, one of the limiting factors in the dosage which would be applied to trees was the sensitiveness of the trees to the hydrocyanic acicL If too high a concentration was obtained, injury to fruit or foliage resulted. In my experiments I have determined that the concentration of hydrocyanic acid about the tree is not the sole factor which causes injury thereto but it is a combination of the concen tration and the time of exposure. My experiments have shown that if a high concentration, say two or three times the normal, is used for a time or respectively, of the normal, the fruit and foliage are subjected to no greater hazard than in normal fumigation. I have found that if such a high concentration is used for a correspondingly shorter period of time the kill of scale is very high, materially higher than that obtained by normal procedure in fumigation.

In practicing my invention I place a tent over the tree to be fumigated as usual and I may then inject the hydrocyanic acid under the tent in any one of several ways of which the following are illustrative:

1. I may utilize a 200% dosage for twenty minutes or a 300% dosage for ten to fifteen minutes. In using these dosagesI am assuming that under the old procedure the 100% schedule is used for forty-five minutes to sixty minutes. I may use, in general, a dosage materially greater than 100%, say from 125% up, for a sufficiently shorter time. If the old schedule varies both in the .dosage and the time, my schedule will also vary in the same manner.

2. I may inject under the tent the normal dosage of hydrocyanic acid under such conditions that diffusion is rapid and is substantially complete in less than five minutes, and preferably in /2 to 1 minute. This may be obtained by the use of a blower either power-driven or hand-driven, which causes a circulation of the gases under the tent so as to quickly make the concentration of hydrocyanic acid gas uniform throughout the space. A sufficiently high rate of diffusion maybe obtained under natural con- .ditions without artificial means if the temperature of the ground under the tent is materially higher than the temperature of the surrounding atmosphere, say 10 to 15 higher. This causes convection currents to provide the uniform concentration at a somewhat slower rate than if artificial means were usedbut in many cases at a satisfactory rate. However, artificial distribution of the gas has consistently shown improvements in kill over unassisted distribution.

3. Instead of injecting the liquid hydrocyanic acid under the tent by a cold apparatus and allowing it to vaporize naturally, I may use a device for injecting and vaporizing the liquid hydrocyanic acid which is artificially heated, such a paratus being well known in the art. However, instead of the ordinary ,openended hose which has heretofore been used in apparatus of this character, I provide a special form of nozzle which contains a plurality of outlets, some of which are horizontally placed, others vertically and still others at such angles as desired. A multiple nozzle of this type directs the currents of gas to all portions of the space and causes rapid diffusion and a uniformly high initial concentration of hydrocyanic acid gas throughout the space.

4. In connection with the methods 2 and I may mean increased dosage for a shorter length of time and, in fact, my best results have been obtained by using a combination of either methods 2 or 3 with method 1. In other words, by using a considerably higher dosage for a shorter time with additional means, either with or without applied heat, for rapidly diifusing the gas throughout the space in, say, less than a minute I obtain uniformly kills from 98 to 100 per cent.

I have also found that in fumigating a grove of trees when the wind is blowing to even a slight extent, it is necessary to take precautions such as will prevent hydrocyanic acid gas from a fumigated tree blowing over to the trees about to be fumigated, and thereby causing stupefaction with consequent unsatisfactory kill when the latter trees are fumigated shortly thereafter. Therefore, in fumigating a grove of trees I prefer to work into the wind, starting with the trees farthest from the direction from which the wind is blowing, and advancing from row to row in the direction from which the wind is blowing. Otherwise, I may adopt various mechanical devices such as tents or other coverings to protect the trees to be fumigated from the drift gas.

By the practice of my new method, not only is it possible to obtain results in the killing of the resistant scale without injury to the trees such as are not obtainable by the customary method, but the new method also increases the efiiciency of fumigation of the ordinary nonresistant scale. Where increased dosages are used the time of treatment is decreased making possible a more rapid fumigation of groves with the resultant decrease in the cost of fumigation.

Although I have described my invention setting forth several specific Ways in which my method may be practiced, it is perfectly apparent that these procedures are not the only procedures which may embody my ideas. Other methods may be used, for example a modification of the old pot method of generation of hot gas under the tent, the use of gas from the now obsolete portable generator for generating hot gas outside of the tent, the use of calcium cyanide as dust, or other cyanides or cyanogen containing material capable of liberating hydrocyanic acid under the conditions of fumigation, or by any other suitable methods. It will be noted that my invention is not at all limited but is broadly the idea of causing a quick and high initial concentration of hydrocyanic acid gas in the space to be fumigated, said concentration being toxic to the insects to be exterminated and above such a concentration as would result in mere stupefaction thereof. My invention is, therefore, not to be limited except as set forth in the claims appended hereto.

What I claim is: r

l. The process of fumigating a tree in an enclosed space which comprises rapidly diffusing throughout the space a dosage of hydrocyanic acid sufficiently heavy and uniformly diffused to obtain a quick kill without stupefaction of the insects to be killed.

2. In a process of fumigating a grove of trees by enclosing each of. the trees in a row and, after fumigation, advancing to fumigate the next row, the step which comprises fumigating the trees in the rows successively. to windward to avoid stupefaction of insects on the next adjacent row of trees to be fumigated.

3. In a process of fumigating a grove of trees by enclosing each of the trees in a row and, after fumigation, advancing to fumigate the next row, the step which comprises fumigating the trees in the rows successively, while preventing stupefactionof insects on the next adjacent row of trees to be fumigated.

4. In a process of fumigating a grove of trees by enclosing each of the trees in a row and, after fumigation, advancing to fumigate the next row,

the step which comprises fumigating the trees in the rows successively, while preventing stupefaction of insects on the next adjacent row of trees to be fumigated by keeping these trees covered.

5. The process of killing insects by fumigation 

